ZAGREB, Yugoslavia — Rebellious Serbs erected roadblocks around towns in the republic of Croatia on Tuesday, sharpening the crisis between Yugoslavia's two biggest ethnic groups that has raised the specter of civil war.

As tension mounted after Sunday's bloody shoot-out between Serbs and Croatian police in the resort of Plitvice, Croatia's nationalist leaders vowed not to cede any territory to their rivals.

"We won't let a single inch of Croatian land be taken from Croatia," the republic's president, Franjo Tudjman, said in a speech in Zagreb, the Croatian capital.

The two men were reacting to a declaration by Serbian leaders in Croatia's Krajina region that they seek to unify the area with Serbia.

Krajina, which includes Plitvice, is a Serbian-populated strip of land in the west of Croatia that has become a flash point for ethnic violence.

Serbs, who make up 11% of Croatia's 4.5 million people, want to separate from the republic because they fear persecution. They also oppose Croatian moves towards secession and say they want to remain in Yugoslavia.

Serbs began sealing off several towns across Croatia on Tuesday by blocking roads with rocks and trees.